Tutorial HPLC

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HPLC - High Performance Liquid ChromatographyWhat Is HPLC (High Performance Liquid Chromatography)?

Brief History and DefinitionLiquid chromatography was defined in the early 1900s by the work of the Russian botanist,

Mikhail S. Tswett. His pioneering studies focused on separating compounds [leaf pigments],

extracted from plants using a solvent, in a column packed with particles.

Tswett filled an open glass column with particles. Two specific materials that he found useful

were powdered chalk [calcium carbonate] and alumina. He poured his sample [solvent extract of

homogenized plant leaves] into the column and allowed it to pass into the particle bed. This was

followed by pure solvent. As the sample passed down through the column by gravity, different

colored bands could be seen separating because some components were moving faster than

others. He related these separated, different-colored bands to the different compounds that

were originally contained in the sample. He had created an analytical separation of these

compounds based on the differing strength of each compounds chemical attraction to theparticles. The compounds that were more strongly attracted to the particlesslowed down, while

other compounds more strongly attracted to the solvent moved faster. This process can be

described as follows: the compounds contained in the sample distribute, or partition differently

between the moving solvent, called the mobile phase, and the particles, called the stationary

phase. This causes each compound to move at a different speed, thus creating a separation of

the compounds.

Tswett coined the name chromatography[from the Greek words chroma, meaning color,

and graph, meaning writingliterally, color writing] to describe his colorful experiment.

[Curiously, the Russian name Tswett means color.] Today, liquid chromatography, in its various

forms, has become one of the most powerful tools in analytical chemistry.

Figure A: Tswett's Experiment


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Liquid Chromatography (LC) Techniques

Liquid chromatography can be performed using planar [Techniques 1 and 2] or column

techniques [Technique 3]. Column liquid chromatography is the most powerful and has the

highest capacity for sample. In all cases, the sample first must be dissolved in a liquid that is

then transported either onto, or into, the chromatographic device.

Technique 1. The sample is spotted onto, and then flows through, a thin layer ofchromatographic particles [stationary phase] fixed onto the surface of a glass plate [Figure B].

The bottom edge of the plate is placed in a solvent. Flow is created by capillary action as the

solvent [mobile phase] diffuses into the dry particle layer and moves up the glass plate. This

technique is called thin-layer chromatography or TLC.

Figure B: Thin-layer Chromatography

Note that the blacksample is a mixture of FD&C yellow, red and blue food dyes that has been

chromatographically separated.

Technique 2. In Figure C, samples are spotted onto paper [stationary phase]. Solvent [mobile

phase] is then added to the center of the spot to create an outward radial flow. This is a form of

paper chromatography. [Classic paper chromatography is performed in a manner similar to that

of TLC with linear flow.] In the upper image, the same black FD&C dye sample is applied to the

paper.


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Figure C: Paper Chromatography

Notice the difference in separation power for this particular paper when compared to the TLC

plate. The green ring indicates that the paper cannot separate the yellow and blue dyes from

each other, but it could separate those dyes from the red dyes. In the bottom image, a green

sample, made up of the same yellow and blue dyes, is applied to the paper. As you would

predict, the paper cannot separate the two dyes. In the middle, a purple sample, made up of red

and blue dyes, was applied to the paper. They are well separated.

Technique 3. In this, the most powerful approach, the sample passes through a column or a

cartridge device containing appropriate particles [stationary phase]. These particles are called

the chromatographic packing material. Solvent [mobile phase] flows through the device. In

solid-phase extraction [SPE], the sample is loaded onto the cartridge and the solvent stream

carries the sample through the device. As in Tswetts experiment, the compounds in the sample

are then separated by traveling at different individual speeds through the device. Here

the blacksample is loaded onto a cartridge. Different solvents are used in each step to create

the separation.


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Figure D-1: Column Chromatography Solid-Phase Extraction [SPE]

When the cartridge format is utilized, there are several ways to achieve flow. Gravity or vacuum

can be used for columns that are not designed to withstand pressure. Typically, the particles in

this case are larger in diameter [> 50 microns] so that there is less resistance to flow. Open

glass columns [Tswetts experiment] are an example of this. In addition, small plastic columns,

typically in the shape of syringe barrels, can be filled with packing-material particles and used toperform sample preparation. This is called solid-phase extraction [SPE]. Here, the

chromatographic device, called a cartridge, is used, usually with vacuum-assisted flow, to clean

up a very complex sample before it is analyzed further.

Smaller particle sizes [


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High performance liquid chromatography is now one of the most powerful tools in analytical

chemistry. It has the ability to separate, identify, and quantitate the compounds that are present

in any sample that can be dissolved in a liquid. Today, compounds in trace concentrations as low

asparts per trillion [ppt] may easily be identified. HPLC can be, and has been, applied to just

about any sample, such as pharmaceuticals, food, nutraceuticals, cosmetics, environmental

matrices, forensic samples, and industrial chemicals.

Figure D-2: HPLC Column

What Is UltraPerformance Liquid Chromatography (UPLC Technology)?

In 2004, further advances in instrumentation and column technology were made to achieve very

significant increases in resolution, speed, and sensitivity in liquid chromatography. Columns with

smaller particles [1.7 micron] and instrumentation with specialized capabilities designed to

deliver mobile phase at 15,000 psi [1,000 bar] were needed to achieve a new level ofperformance. A new system had to be holistically created to perform ultra-performance liquid

chromatography, now known as UPLC technology.

Basic research is being conducted today by scientists working with columns containing even

smaller 1-micron-diameter particles and instrumentation capable of performing at 100,000 psi

[6,800 bar]. This provides a glimpse of what we may expect in the future.

How Does High Performance Liquid Chromatography Work?

The components of a basic high-performance liquid chromatography [HPLC] system are shown in

the simple diagram in Figure E.

A reservoir holds the solvent [called the mobile phase, because it moves]. A high-pressure pump

[solvent delivery system or solvent manager] is used to generate and meter a specified flow rate

of mobile phase, typically milliliters per minute. An injector [sample manager or autosampler] is

able to introduce [inject] the sample into the continuously flowing mobile phase stream that

carries the sample into the HPLC column. The column contains the chromatographic packing

material needed to effect the separation. This packing material is called the stationary phase

because it is held in place by the column hardware. A detector is needed to see the separated

compound bands as they elute from the HPLC column [most compounds have no color, so we

cannot see them with our eyes]. The mobile phase exits the detector and can be sent to waste,

or collected, as desired. When the mobile phase contains a separated compound band, HPLC

provides the ability to collect this fraction of the eluate containing that purified compound for

further study. This is called preparative chromatography [discussed in the section on HPLC

Scale].

Note that high-pressure tubing and fittings are used to interconnect the pump, injector, column,

and detector components to form the conduit for the mobile phase, sample, and separated

compound bands.


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Figure F: A Typical HPLC [Waters Alliance] System

HPLC Operation

A simple way to understand how we achieve the separation of the compounds contained in a

sample is to view the diagram in Figure G.

Mobile phase enters the column from the left, passes through the particle bed, and exits at the

right. Flow direction is represented by green arrows. First, consider the top image; it representsthe column at time zero [the moment of injection], when the sample enters the column and

begins to form a band. The sample shown here, a mixture of yellow, red, and blue dyes, appears

at the inlet of the column as a single black band. [In reality, this sample could be anything that

can be dissolved in a solvent; typically the compounds would be colorless and the column wall

opaque, so we would need a detector to see the separated compounds as they elute.]

After a few minutes [lower image], during which mobile phase flows continuously and steadily

past the packing material particles, we can see that the individual dyes have moved in separate

bands at different speeds. This is because there is a competition between the mobile phase and

the stationary phase for attracting each of the dyes or analytes. Notice that the yellow dye band

moves the fastest and is about to exit the column. The yellow dye likes [is attracted to] the

mobile phase more than the other dyes. Therefore, it moves at a fasterspeed, closer to that ofthe mobile phase. The blue dye band likes the packing material more than the mobile phase. Its

stronger attraction to the particles causes it to move significantlyslower. In other words, it is the

most retained compound in this sample mixture. The red dye band has an intermediate

attraction for the mobile phase and therefore moves at an intermediate speed through the

column. Since each dye band moves at different speed, we are able to separate it

chromatographically.


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Figure G: Understanding How a Chromatographic Column Works Bands

What Is a Detector?

As the separated dye bands leave the column, they pass immediately into the detector. The

detector contains a flow cell that sees [detects] each separated compound band against a

background of mobile phase [see Figure H]. [In reality, solutions of many compounds at typical

HPLC analytical concentrations are colorless.] An appropriate detector has the ability to sense

the presence of a compound and send its corresponding electrical signal to a computer data

station. A choice is made among many different types of detectors, depending upon the

characteristics and concentrations of the compounds that need to be separated and analyzed, as

discussed earlier.

What Is a Chromatogram?

A chromatogram is a representation of the separation that has chemically [chromatographically]

occurred in the HPLC system. A series of peaks rising from a baseline is drawn on a time axis.

Each peak represents the detector response for a different compound. The chromatogram is

plotted by the computer data station [see Figure H].

Figure H: How Peaks Are Created


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In Figure H, the yellow band has completely passed through the detector flow cell; the electrical

signal generated has been sent to the computer data station. The resulting chromatogram has

begun to appear on screen. Note that the chromatogram begins when the sample was first

injected and starts as a straight line set near the bottom of the screen. This is called the

baseline; it represents pure mobile phase passing through the flow cell over time. As the yellow

analyte band passes through the flow cell, a stronger signal is sent to the computer. The line

curves, first upward, and then downward, in proportion to the concentration of the yellow dye inthe sample band. This creates a peak in the chromatogram. After the yellow band passes

completely out of the detector cell, the signal level returns to the baseline; the flow cell now has,

once again, only pure mobile phase in it. Since the yellow band moves fastest, eluting first from

the column, it is the first peak drawn.

A little while later, the red band reaches the flow cell. The signal rises up from the baseline as

the red band first enters the cell, and the peak representing the red band begins to be drawn. In

this diagram, the red band has not fully passed through the flow cell. The diagram shows what

the red band and red peak would look like if we stopped the process at this moment. Since most

of the red band has passed through the cell, most of the peak has been drawn, as shown by the

solid line. If we could restart, the red band would completely pass through the flow cell and the

red peak would be completed [dotted line]. The blue band, the most strongly retained, travels atthe slowest rate and elutes after the red band. The dotted line shows you how the completed

chromatogram would appear if we had let the run continue to its conclusion. It is interesting to

note that the width of the blue peak will be the broadest because the width of the blue analyte

band, while narrowest on the column, becomes the widest as it elutes from the column. This is

because it moves more slowly through the chromatographic packing material bed and requires

more time [and mobile phase volume] to be eluted completely. Since mobile phase is

continuously flowing at a fixed rate, this means that the blue band widens and is more dilute.

Since the detector responds in proportion to the concentration of the band, the blue peak is

lower in height, but larger in width.

Identifying and Quantitating Compounds

In Figure H, three dye compounds are represented by three peaks separated in time in the

chromatogram. Each elutes at a specific location, measured by the elapsed time between the

moment of injection [time zero] and the time when the peak maximum elutes. By comparing

each peaks retention time [tR] with that of injected reference standards in the same

chromatographic system [same mobile and stationary phase], a chromatographer may be able

to identify each compound.


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Figure I-1: Identification

In the chromatogram shown in Figure I-1, the chromatographer knew that, under these LC

system conditions, the analyte, acrylamide, would be separated and elute from the column at

2.85 minutes [retention time]. Whenever a new sample, which happened to contain acrylamide,

was injected into the LC system under the same conditions, a peak would be present at 2.85

minutes [see Sample B in Figure I-2].

[For a better understanding of why some compounds move more slowly [are better retained]

than others, please review the HPLC Separation Modes section on page 28].

Once identity is established, the next piece of important information is how much of each

compound was present in the sample. The chromatogram and the related data from the detector

help us calculate the concentration of each compound. The detector basically responds to theconcentration of the compound band as it passes through the flow cell. The more concentrated it

is, the stronger the signal; this is seen as a greater peak height above the baseline.


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Figure I-2: Identification and Quantitation

In Figure I-2, chromatograms for Samples A and B, on the same time scale, are stacked one

above the other. The same volume of sample was injected in both runs. Both chromatograms

display a peak at a retention time [tR] of 2.85 minutes, indicating that each sample contains

acrylamide. However, Sample A displays a much bigger peak for acrylamide. The area under a

peak [peak area count] is a measure of the concentration of the compound it represents. This

area value is integrated and calculated automatically by the computer data station. In this

example, the peak for acrylamide in Sample A has 10 times the area of that for Sample B. Using

reference standards, it can be determined that Sample A contains 10 picograms of acrylamide,

which is ten times the amount in Sample B [1 picogram]. Note there is another peak [not

identified] that elutes at 1.8 minutes in both samples. Since the area counts for this peak in both

samples are about the same, this unknown compound may have the same concentration in both

samples.

Isocratic and Gradient LC System Operation

Two basic elution modes are used in HPLC. The first is called isocratic elution. In this mode, the

mobile phase, either a pure solvent or a mixture, remains the same throughout the run. A

typical system is outlined in Figure J-1.


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Figure J-1: Isocratic LC System

The second type is called gradient elution, wherein, as its name implies, the mobile phase

composition changes during the separation. This mode is useful for samples that contain

compounds that span a wide range of chromatographic polarity [see section on HPLC Separation

Modes]. As the separation proceeds, the elution strength of the mobile phase is increased to

elute the more strongly retained sample components.

Figure J-2: High-Pressure-Gradient System

In the simplest case, shown in Figure J-2, there are two bottles of solvents and two pumps. The

speed of each pump is managed by the gradient controller to deliver more or less of each


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solvent over the course of the separation. The two streams are combined in the mixer to create

the actual mobile phase composition that is delivered to the column over time. At the beginning,

the mobile phase contains a higher proportion of the weaker solvent [Solvent A]. Over time, the

proportion of the stronger solvent [Solvent B] is increased, according to a predetermined

timetable. Note that in Figure J-2, the mixer is downstream of the pumps; thus the gradient is

created under high pressure. Other HPLC systems are designed to mix multiple streams of

solvents under low pressure, ahead of a single pump. A gradient proportioning valve selectsfrom the four solvent bottles, changing the strength of the mobile phase over time [see Figure J-

3].

Figure J-3: Low-Pressure-Gradient System

HPLC Scale [Analytical, Preparative, and Process]

We have discussed how HPLC provides analytical data that can be used both to identify and to

quantify compounds present in a sample. However, HPLC can also be used to purify and collect

desired amounts of each compound, using a fraction collector downstream of the detector flow

cell. This process is called preparative chromatography [see Figure K].

In preparative chromatography, the scientist is able to collect the individual analytes as they

elute from the column [e.g., in this example: yellow, then red, then blue].


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Figure K: HPLC System for Purification: Preparative Chromatography

The fraction collector selectively collects the eluate that now contains a purified analyte, for aspecified length of time. The vessels are moved so that each collects only a single analyte peak.

A scientist determines goals for purity level and amount. Coupled with knowledge of the

complexity of the sample and the nature and concentration of the desired analytes relative to

that of the matrix constituents, these goals, in turn, determine the amount of sample that needs

to be processed and the required capacity of the HPLC system. In general, as the sample size

increases, the size of the HPLC column will become larger and the pump will need higher

volume-flow-rate capacity. Determining the capacity of an HPLC system is called selecting the

HPLC scale. Table A lists various HPLC scales and their chromatographic objectives.

Table A: Chromatography Scale

The ability to maximize selectivity with a specific combination of HPLC stationary and mobile

phasesachieving the largest possible separation between two sample components of interest

is critical in determining the requirements for scaling up a separation [see discussion on HPLC

Separation Modes]. Capacity then becomes a matter of scaling the column volume [Vc] to the

amount of sample to be injected and choosing an appropriate particle size [determines pressure

and efficiency; see discussion of Separation Power]. Column volume, a function of bed length [L]

and internal diameter [i.d.], determines the amount of packing material [particles] that can be

contained (see Figure L).


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Figure L: HPLC Column Dimensions

In general, HPLC columns range from 20 mm to 500 mm in length [L] and 1 mm to 100 mm in

internal diameter [i.d.]. As the scale of chromatography increases, so do column dimensions,

especially the cross-sectional area. To optimize throughput, mobile phase flow rates must

increase in proportion to cross-sectional area. If a smaller particle size is desirable for more

separation power, pumps must then be designed to sustain higher mobile-phase-volume flow

rates at high backpressure. Table B presents some simple guidelines on selecting the column i.d.

and particle size range recommended for each scale of chromatography.

For example, a semi-preparative-scale application [red X] would use a column with an internal

diameter of 1040 mm containing 515 micron particles. Column length could then be calculated

based on how much purified compound needs to be processed during each run and on how much

separation power is required.

Table B: Chromatography Scale vs. Column Diameter and Particle Size


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HPLC Column Hardware

A column tube and fittings must contain the chromatographic packing material [stationary

phase] that is used to effect a separation. It must withstand backpressure created both during

manufacture and in use. Also, it must provide a well-controlled [leak-free, minimum-volume,

and zero-dead-volume] flow path for the sample at its inlet, and analyte bands at its outlet, and

be chemically inert relative to the separation system [sample, mobile, and stationary phases].

Most columns are constructed of stainless steel for highest pressure resistance. PEEK[an

engineered plastic] and glass, while less pressure tolerant, may be used when inert surfaces are

required for special chemical or biological applications. [Figure M-1].

Figure M-1: Column Hardware Examples

A glass column wall offers a visual advantage. In the photo in Figure M-2, flow has been stopped

while the sample bands are still in the column. You can see that the three dyes in the injected

sample mixture have already separated in the bed; the yellow analyte, traveling fastest, is just

about to exit the column.

Figure M-2: A Look Inside a Column

Separation Performance Resolution

The degree to which two compounds are separated is called chromatographic resolution [RS].

Two principal factors that determine the overall separation power or resolution that can be


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achieved by an HPLC column are: mechanical separation power, created by the column length,

particle size, and packed-bed uniformity, and chemical separation power, created by the

physicochemical competition for compounds between the packing material and the mobile phase.

Efficiency is a measure of mechanical separation power, while selectivity is a measure of

chemical separation power.

Mechanical Separation Power EfficiencyIf a column bed is stable and uniformly packed, its mechanical separation power is determined

by the column length and the particle size. Mechanical separation power, also called efficiency, is

often measured and compared by a plate number [symbol = N]. Smaller-particle

chromatographic beds have higher efficiency and higher backpressure. For a given particle size,

more mechanical separation power is gained by increasing column length. However, the trade-

offs are longer chromatographic run times, greater solvent consumption, and higher

backpressure. Shorter column lengths minimize all these variables but also reduce mechanical

separation power, as shown in Figure N.

Figure N: Column Length and Mechanical Separating Power [Same Particle Size]

Figure O: Particle Size and Mechanical Separating Power [Same Column Length]

For a given particle chemistry, mobile phase, and flow rate, as shown in Figure O, a column of

the same length and i.d., but with a smaller particle size, will deliver more mechanical

separation power in the same time. However, its backpressure will be much higher.


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Another way to think of this is by the familiar analogy: oil [non-polar] and water [polar] dont

mix. Unlike in magnetism where opposite poles attract each other, chromatographic separations

based on polarity depend upon the stronger attraction between likes and the weaker attraction

between opposites. Remember,Like attracts likein polarity-based chromatography.

Figure Q: Proper Combination of Mobile and Stationary Phases Effects Separation Based on Polarity

To design a chromatographic separation system [see Figure Q], we create competition for the

various compounds contained in the sample by choosing a mobile phase and a stationary phase

with different polarities. Then, compounds in the sample that are similar in polarity to the

stationary phase [column packing material] will be delayed because they are more stronglyattracted to the particles. Compounds whose polarity is similar to that of the mobile phase will

be preferentially attracted to it and move faster.

In this way, based upon differences in the relative attraction of each compound for each phase,

a separation is created by changing the speeds of the analytes.

Figures R-1, R-2, and R-3 display typical chromatographic polarity ranges for mobile phases,

stationary phases, and sample analytes, respectively. Lets consider each in turn to see how a

chromatographer chooses the appropriate phases to develop the attraction competition needed

to achieve a polarity-based HPLC separation.

Figure R-1: Mobile Phase Chromatographic Polarity Spectrum

A scale, such as that shown in Figure R-1, upon which some common solvents are placed in

order of relative chromatographic polarity is called an eluotropic series. Mobile phase molecules

that compete effectively with analyte molecules for the attractive stationary phase sites displace

these analytes, causing them to move faster through the column [weakly retained]. Water is at

the polar end of mobile-phase-solvent scale, while hexane, an aliphatic hydrocarbon, is at the

non-polar end. In between, single solvents, as well as miscible-solvent mixtures [blended in

proportions appropriate to meet specific separation requirements], can be placed in order of


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elution strength. Which end of the scale represents the strongest mobile phase depends upon

the nature of the stationary phase surface where the competition for the analyte molecules

occurs.

Figure R-2: Stationary Phase Particle Chromatographic Polarity Spectrum

Silica has an active, hydrophilic [water-loving] surface containing acidic silanol [silicon-

containing analog of alcohol] functional groups. Consequently, it falls at the polar end of the

stationary-phase scale shown in Figure R-2. The activity or polarity of the silica surface may be

modified selectively by chemically bonding to it less polar functional groups [bonded phase].

Examples shown here include, in order of decreasing polarity, cyanopropylsilyl- [CN], n-

octylsilyl- [C8], and n-octadecylsilyl- [C18, ODS] moieties on silica. The latter is a hydrophobic

[water-hating], very non-polar packing.

Figure R-3: Compound/Analyte Chromatographic Polarity Spectrum

Figure R-3 repeats the chromatographic polarity spectrum of our sample [shown in Figure P].

After considering the polarity of both phases, then, for a given stationary phase, a

chromatographer must choose a mobile phase in which the analytes of interest are retained, but

not so strongly that they cannot be eluted. Among solvents of similar strength, the

chromatographer considers which phase combination may best exploit the more subtle

differences in analyte polarity and solubility to maximize the selectivity of the chromatographic

system. Like attracts like, but, as you probably can imagine from the discussion so far, creating

a separation based upon polarity involves knowledge of the sample and experience with various

kinds of analytes and retention modes. To summarize, the chromatographer will choose the best

combination of a mobile phase and particle stationary phase with appropriately opposite

polarities. Then, as the sample analytes move through the column, the rule like attracts like will

determine which analytes slow down and which proceed at a faster speed.


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Normal-Phase HPLC

In his separations of plant extracts, Tswett was successful using a polar stationary phase [chalk

in a glass column; see Figure A] with a much less polar [non-polar] mobile phase. This classical

mode of chromatography became known as normal phase.

Figure S-1: Normal-Phase Chromatography

Figure S-1 represents a normal-phase chromatographic separation of our three-dye test mixture.

The stationary phase is polar and retains the polar yellow dye most strongly. The relatively non-

polar blue dye is won in the retention competition by the mobile phase, a non-polar solvent, and

elutes quickly. Since the blue dye is most like the mobile phase [both are non-polar], it moves

faster. It is typical for normal-phase chromatography on silica that the mobile phase is 100%

organic; no water is used.

Reversed-Phase HPLC

The term reversed-phase describes the chromatography mode that is just the opposite of normal

phase, namely the use of a polar mobile phase and a non-polar [hydrophobic] stationary phase.

Figure S-2 illustrates the black three-dye mixture being separated using such a protocol.

Figure S-2: Reversed-Phase Chromatography

Now the most strongly retained compound is the more non-polar blue dye, as its attraction to

the non-polar stationary phase is greatest. The polar yellow dye, being weakly retained, is won

in competition by the polar, aqueous mobile phase, moves the fastest through the bed, and

elutes earliest like attracts like.

Today, because it is more reproducible and has broad applicability, reversed-phase

chromatography is used for approximately 75% of all HPLC methods. Most of these protocols use


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as the mobile phase an aqueous blend of water with a miscible, polar organic solvent, such as

acetonitrile or methanol. This typically ensures the proper interaction of analytes with the non-

polar, hydrophobic particle surface. A C18bonded silica [sometimes called ODS] is the most

popular type of reversed-phase HPLC packing.

Table C presents a summary of the phase characteristics for the two principal HPLC separation

modes based upon polarity. Remember, for these polarity-based modes,like attracts like.

Table C: Phase Characteristics for Separations Based on Polarity

Hydrophilic-Interaction Chromatography [HILIC]

HILIC may be viewed as a variant of normal-phase chromatography. In normal-phase

chromatography, the mobile phase is 100% organic. Only traces of water are present in the

mobile phase and in the pores of the polar packing particles. Polar analytes bind strongly to the

polar stationary phase and may not elute.

Adding some water [< 20%] to the organic mobile phase [typically an aprotic solvent like

acetonitrile] makes it possible to separate and elute polar compounds that are strongly retained

in the normal-phase mode [or weakly retained in the reversed-phase mode]. Water, a very polar

solvent, competes effectively with polar analytes for the stationary phase. HILIC may be run in

either isocratic or gradient elution modes. Polar compounds that are initially attracted to thepolar packing material particles can be eluted as the polarity [strength] of the mobile phase is

increased [by adding more water]. Analytes are eluted in order of

increasinghydrophilicity[chromatographic polarity relative to water]. Buffers or salts may be

added to the mobile phase to keep ionizable analytes in a single form.

Hydrophobic-Interaction Chromatography [HIC]

HIC is a type of reversed-phase chromatography that is used to separate large biomolecules,

such as proteins. It is usually desirable to maintain these molecules intact in an aqueous

solution, avoiding contact with organic solvents or surfaces that might denature them. HIC takes

advantage of the hydrophobic interaction of large molecules with a moderately hydrophobic

stationary phase, e.g., butyl-bonded [C4], rather than octadecyl-bonded [C18], silica. Initially,

higher salt concentrations in water will encourage the proteins to be retained [salted out] on thepacking. Gradient separations are typically run by decreasing salt concentration. In this way,

biomolecules are eluted in order of increasing hydrophobicity.

Separations Based on Charge: Ion-Exchange Chromatography [IEC]

For separations based on polarity, like is attracted to like and opposites may be repelled. In ion-

exchange chromatography and other separations based upon electrical charge, the rule is

reversed. Likes may repel, while opposites are attracted to each other. Stationary phases for

ion-exchange separations are characterized by the nature and strength of the acidic or basic


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functions on their surfaces and the types of ions that they attract and retain. Cation exchange is

used to retain and separate positively charged ions on a negative surface.

Conversely, anion exchange is used to retain and separate negatively charged ions on

apositive surface [see Figure T]. With each type of ion exchange, there are at least two general

approaches for separation and elution.

Figure T: Ion-Exchange Chromatography

Strong ion exchangers bear functional groups [e.g., quaternary amines or sulfonic acids] that

are always ionized. They are typically used to retain and separate weakions. These weak ions

may be eluted by displacement with a mobile phase containing ions that are more strongly

attracted to the stationary phase sites. Alternately, weak ions may be retained on the column,

then neutralizedby in situchanging the pH of the mobile phase, causing them to lose their

attraction and elute.

Weakion exchangers [e.g., with secondary-amine or carboxylic-acid functions] may be

neutralized above or below a certain pH value and lose their ability to retain ions by charge.

When charged, they are used to retain and separate strong ions. If these ions cannot be eluted

by displacement, then the stationary phase exchange sites may be neutralized, shutting offthe

ionic attraction, and permitting elution of the charged analytes.


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Table D: Ion-Exchange Guidelines

When weak ion exchangers are neutralized, they may retain and separate species

by hydrophobic[reversed-phase] or hydrophilic[normal-phase] interactions; in these cases,elution strength is determined by the polarity of the mobile phase [Figure R-1]. Thus, weak ion

exchangers may be used for mixed-mode separations [separations based on both polarity and

charge].

Table D outlines guidelines for the principal categories of ion exchange. For example, to retain

a strongly basicanalyte [always positively charged], use a weak-cation-exchange stationary

phase particle at pH > 7; this assures a negativelycharged particle surface. To release or elute

the strong base, lower the pH of the mobile phase below 3; this removes the surface charge

and shuts offthe ion-exchange retention mechanism.

Note that a pKa is the pH value at which 50% of the functional group is ionized and 50% is

neutral. To assure an essentially neutral, or a fully charged, analyte or particle surface, the pH

must be adjusted to a value at least 2 units beyond the pKa, as appropriate [indicated in Table

D].

Do not use a strong-cation exchanger to retain a strong base; both remain charged and strongly

attracted to each other, making the base nearly impossible to elute. It can only be removed by

swamping the strong cation exchanger with a competing base that exhibits even stronger

retention and displaces the compound of interest by winning the competition for the active


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exchange sites. This approach is rarely practical, or safe, in HPLC and SPE. [Very strong acids

and bases are dangerous to work with, and they may be corrosive to materials of construction

used in HPLC fluidics!]

Separations Based on Size: Size-Exclusion Chromatography [SEC]

Gel-Permeation Chromatography [GPC]

In the 1950s, Porath and Flodin discovered that biomolecules could be separated based on their

size, rather than on their charge or polarity, by passing, or filtering, them through a controlled-

porosity, hydrophilic dextran polymer. This process was termed gel filtration. Later, an

analogous scheme was used to separate synthetic oligomers and polymers using organic-

polymer packings with specific pore-size ranges. This process was called gel-permeation

chromatography [GPC]. Similar separations done using controlled-porosity silica packings were

called size-exclusion chromatography [SEC]. Introduced in 1963, the first commercial HPLC

instruments were designed for GPC applications [see Reference 3].

All of these techniques are typically done on stationary phases that have been synthesized with

a pore-size distribution over a range that permits the analytes of interest to enter, or to be

excluded from, more or less of the pore volume of the packing. Smaller molecules penetrate

more of the pores on their passage through the bed. Larger molecules may only penetrate pores

above a certain size so they spend less time in the bed. The biggest molecules may be totally

excluded from pores and pass only between the particles, eluting very quickly in a small volume.

Mobile phases are chosen for two reasons: first, they are good solvents for the analytes; and,

second, they may prevent any interactions [based on polarity or charge] between the analytes

and the stationary phase surface. In this way, the larger molecules elute first, while the smaller

molecules travel slower [because they move into and out of more of the pores] and elute later,

in decreasing order of their size in solution. Hence the simple rule: Big ones come out first.

Since it is possible to correlate the molecular weight of a polymer with its size in solution, GPC

revolutionized measurement of the molecular-weight distribution of polymers that, in turn,

determines the physical characteristics that may enhance, or detract from, polymer processing,

quality, and performance [how to tell goodfrombadpolymer].

Conclusion

We hope you have enjoyed this brief introduction to HPLC. We encourage you to read the

references below and to study the Appendix on HPLC Nomenclature.

Appendix: HPLC Nomenclature

*Indicates a definition adapted from: L.S. Ettre, Nomenclature for Chromatography, Pure Appl.Chem. 65: 819-872 [1993], 1993 IUPAC; an updated version of this comprehensive report is

available in the Orange Book, Chapter 9: Separations [1997] at:

.

Alumina

A porous, particulate form of aluminum oxide [Al203] used as a stationary phase in normal-

phase adsorption chromatography. Alumina has a highly active basic surface; the pH of a 10%

aqueous slurry is about 10. It is successively washed with strong acid to make neutral and acidic


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grades [slurry pH 7.5 and 4, resp.]. Alumina is more hygroscopic than silica. Its activity is

measured according to the Brockmann scale for water content; e.g., Activity Grade I contains

1% H2O.

H. Brockmann and H. Schodder, Ber. 74: 73 (1941).

Baseline*

The portion of the chromatogram recording the detector response when only the mobile phase

emerges from the column.

Cartridge

A type of column, without endfittings, that consists simply of an open tube wherein the packing

material is retained by a frit at either end. SPE cartridges may be operated in parallel on a

vacuum-manifold. HPLC cartridges are placed into a cartridge holder that has fluid connections

built into each end. Cartridge columns are easy to change, less expensive, and more convenient

than conventional columns with integral endfittings.

Chromatogram*

A graphical or other presentation of detector response or other quantity used as a measure of

the concentration of the analyte in the effluent versus effluent volume or time. In planar

chromatography [e.g., thin-layer chromatography or paper

chromatography], chromatogram may refer to the paper or layer containing the separated

zones.

Chromatography*

A dynamic physicochemical method of separation in which the components to be separated are

distributed between two phases, one of which is stationary [thestationary phase] while the other

[the mobile phase] moves relative to the stationary phase.

Column Volume* [Vc]

The geometric volume of the part of the tube that contains the packing [internal cross-sectional

area of the tube multiplied by the packed bed length, L]. Theinterparticle volume of the column,

also called the interstitial volume, is the volume occupied by the mobile phase between the

particles in the packed bed. The void volume [V0] is the total volume occupied by the mobile

phase, i.e. the sum of the interstitial volume and the intraparticle volume [also calledpore

volume].

Detector* [see Sensitivity]

A device that indicates a change in the composition of the eluent by measuring physical or

chemical properties [e.g., UV/visible light absorbance, differential refractive index, fluorescence,

or conductivity]. If the detectors response is linear with respect to sample concentration, then,

by suitable calibration with standards, the amount of a component may be quantitated. Often, it

may be beneficial to use two different types of detectors in series. In this way, more

corroboratory or specific information may be obtained about the sample analytes. Some

detectors [e.g.,electrochemical, mass spectrometric] are destructive; i.e., they effect a chemical

change in the sample components. If a detector of this type is paired with a non-destructive

detector, it is usually placed second in the flow path.

Display

A device that records the electrical response of a detector on a computer screen in the form of a


27/34

chromatogram. Advanced data recording systems also perform calculations using sophisticated

algorithms, e.g., to integrate peak areas, subtract baselines, match spectra, quantitate

components, and identify unknowns by comparison to standard libraries.

Efficiency [H, see Plate Number, Resolution, Sensitivity, Speed]

A measure of a columns ability to resist the dispersion of a sample band as it passes through

the packed bed. An efficient column minimizes band dispersion orbandspreading. Higherefficiency is important for effective separation, greater sensitivity, and/or identification of similar

components in a complex sample mixture.

Nobelists Martin and Synge, by analogy to distillation, introduced the concept ofplate height[H,

or H.E.T.P., height equivalent to a theoretical plate] as a measure of chromatographic efficiency

and as a means to compare column performance. Presaging HPLC and UPLC technology, they

recognized that a homogeneous bed packed with the smallest possible particle size [requiring

higher pressure] was key to maximum efficiency. The relation between column and separation

system parameters that affect bandspreading

was later described in an equation by van Deemter.

Chromatographers often refer to a quantity that they can calculate easily and directly from

measurements made on a chromatogram, namelyplate number[N], as efficiency. Plate height is

then determined from the ratio of the length of the column bed to N [H = L/N; methods of

calculating N from a chromatogram are shown in Figure U]. It is important to note that

calculation of N or H using these methods is correct only for isocratic conditions and cannot be

used for gradient separations.

A.J.P. Martin and R.M. Synge, Biochem. J. 35: 1358-1368 [1941]

J.J. van Deemter, F. J. Zuiderweg and A. Klinkenberg, Chem. Eng. Sci. 5: 271-289 [1956]

Eluate

The portion of the eluentthat emerges from the column outlet containing analytes in solution. In

analytical HPLC, the eluate is examined by the detector for the concentration or mass of analytes

therein. In preparative HPLC, the eluate is collected continuously in aliquots at uniform time or

volume intervals, or discontinuously only when a detector indicates the presence of a peak of

interest. These fractions are subsequently processed to obtain purified compounds.

Eluent

The mobile phase [see Elution Chromatography].

Eluotropic Series

A list of solvents ordered by elution strength with reference to specified analytes on a standard

sorbent. Such a series is useful when developing both isocratic and gradient elution methods.

Trappe coined this term after showing that a sequence of solvents of increasing polarity could

separate lipid fractions on alumina. Later, Snyder measured and tabulated solvent strength

parameters for a large list of solvents on several normal-phase LC sorbents. Neher created a

very usefulnomogram by which equi-eluotropic

[constant elution strength] mixtures of normal-phase solvents could be chosen to optimize the

selectivity of TLC separations.

A typical normal-phase eluotropic series would start at the weak end with non-polar aliphatic

hydrocarbons, e.g., pentane or hexane, then progress successively to benzene [an aromatic


28/34

hydrocarbon], dichloromethane [a chlorinated hydrocarbon], diethyl ether, ethyl acetate [an

ester], acetone [a ketone], and, finally, methanol [an alcohol] at the strong end [see Figure R-

1].

W. Trappe, Biochem. Z. 305: 150 [1940]

L. R. Snyder, Principles of Adsorption Chromatography, Marcel Dekker [1968], pp. 192-197

R. Neher in G.B. Marini-Bettlo, ed., Thin-Layer Chromatography, Elsevier [1964] pp. 75-86.

Elute* [verb]

To chromatograph by elution chromatography. The process of elution may be stopped while all

the sample components are still on the chromatographic bed [planar thin-layer or paper

chromatography] or continued until the components have left the chromatographic bed [column

chromatography].

Note: The term elute is preferred to develop [a term used in planar chromatography], to avoid

confusion with the practice of method development, whereby a separation system [the

combination of mobile and stationary phases] is optimized for a particular separation.

Elution Chromatography*

A procedure for chromatographic separation in which the mobile phase is continuously passed

through the chromatographic bed. In HPLC, once the detector baseline has stabilized and the

separation system has reached equilibrium, a finite slug of sample is introduced into the flowing

mobile phase stream. Elution continues until all analytes of interest have passed through the

detector.

Elution Strength

A measure of the affinity of a solvent relative to that of the analyte for the stationary phase. A

weak solvent cannot displace the analyte, causing it to be strongly retained on the stationary

phase. A strong solvent may totally displace all the analyte molecules and carry them through

the column unretained. To achieve a proper balance of effective separation and reasonable

elution volume, solvents are often blended to set up an appropriate competition between the

phases, thereby optimizing both selectivity and

separation time for a given set of analytes [see Selectivity].

Dipole moment, dielectric constant, hydrogen bonding, molecular size and shape, and surface

tension may give some indication of elution strength. Elution strength is also determined by the

separation mode. An eluotropic series of solvents may be ordered by increasing strength in one

direction under adsorption or normal-phaseconditions; that order may be nearly opposite

under reversed-phase partitionconditions [see Figure R-1].

Fluorescence Detector

Fluorescence detectors excite a sample with a specified wavelength of light. This causes certain

compounds to fluoresce and emit light at a higher wavelength. A sensor, set to a

specific emission wavelength and masked so as not to be blinded by the excitation source,

collects only the emitted light. Often analytes that do not natively fluoresce may be derivatized

to take advantage of the high sensitivity and selectivity of this form of

detection, e.g., AccQTag derivatization of amino acids.

Flow Rate*

The volume of mobile phase passing through the column in unit time. In HPLC systems, the flow


29/34

rate is set by the controller for the solvent delivery system [pump]. Flow rate accuracy can be

checked by timed collection and measurement of the effluent at the column outlet. Since a

solvents density varies with temperature, any calibration or flow rate measurement must take

this variable into account. Most accurate determinations are made, when possible, by weight,

not volume.

Uniformity [precision] and reproducibilityof flow rate is important to many LC techniques,especially in separations where retention times are key to analyte identification, or in gel-

permeation chromatographywhere calibration and correlation of retention times are critical to

accurate molecular-weight-distribution measurements of polymers.

Often, separation conditions are compared by means oflinear velocity, not flow rate. The linear

velocity is calculated by dividing the flow rate by the cross-sectional area of the column. While

flow rate is expressed in volume/time [e.g., mL/min], linear velocity is measured in length/time

[e.g., mm/sec].

Gel-Permeation Chromatography*

Separation based mainly upon exclusion effects due to differences in molecular size and/or

shape. Gelpermeation chromatographyand gel filtration chromatographydescribe the process

when the stationary phase is a swollen gel. Both are forms ofsize-exclusion chromatography.

Porath and Flodin first described gel-filtration using dextran gels and aqueous mobile phases for

the size-based separation of biomolecules. Moore applied similar principles to the separation of

organic polymers by size in solution using

organic-solvent mobile phases on porous polystyrene-divinylbenzene polymer gels.

J. Porath, P. Flodin, Nature 183: 1657-1659 [1959]

J.C. Moore, U.S. Patent3,326,875 [filed Jan. 1963; issued June 1967]

Gradient

The change over time in the relative concentrations of two [or more] miscible solvent

components that form a mobile phase of increasing elution strength. A step gradientis typically

used in solid-phase extraction; in each step, the eluent composition is changed abruptly from a

weaker mobile phase to a stronger mobile phase. It is even possible, by drying the SPE sorbent

bed in between steps, to change from one solvent to another immiscible solvent.

A continuous gradient is typically generated by a low- or high-pressure mixing system [see

Figures J-2 and J-3] according to a pre-determined curve [linear or non-linear] representing the

concentration of the stronger solvent B in the initial solvent A over a fixed time period. A holdat

a fixed isocraticsolvent composition can be programmed at any time point within a continuous

gradient. At the end of a separation, the gradient program can also be set to return to the initial

mobile phase composition to re-equilibrate the column in preparation for the injection of the

next sample. Sophisticated HPLC systems can blend as many as four or more solvents [or

solvent mixtures] into a continuous gradient.

Injector [Autosampler, Sample Manager]

A mechanism for accurately and precisely introducing [injecting] a discrete, predetermined

volume of a sample solution into the flowing mobile phase stream. The injector can be a simple

manual device, or a sophisticated autosampler that can be programmed for unattended

injections of many samples from an array of individual vials or wells in a predetermined


30/34

sequence. Sample compartments in these systems may even be temperature controlled to

maintain sample integrity over many hours of operation.

Most modern injectors incorporate some form of syringe-filled sample loop that can be switched

on- or offline by means of a multi-port valve. A well-designed, minimal-internal-volume injection

system is situated as close to the column inlet as possible and minimizes the spreading of the

sample band. Between sample injections, it is also capable of being flushed to waste by mobilephase, or a wash solvent, to prevent carryover[contamination of the present sample by a

previous one].

Samples are best prepared for injection, if possible, by dissolving them in the mobile phase into

which they will be injected; this may prevent issues with separation and/or detection. If another

solvent must be used, it is desirable that its elution strength be equal to or less than that of the

mobile phase. It is often wise to mix a bit of a sample solution with the mobile phase offline to

test for precipitation or miscibility issues that might compromise a successful separation.

Inlet

The end of the column bed where the mobile phase stream and sample enter. A porous, inert frit

retains the packing material and protects the sorbent bed inlet from particulate contamination.

Good HPLC practice dictates that samples and mobile phases should be particulate-free; this

becomes imperative for small-particle columns whose inlets are much more easily plugged. If

the column bed inlet becomes clogged and exhibits higher-than-normal backpressure,

sometimes, reversing the flow direction while directing the effluent to waste may dislodge and

flush out sample debris that sits atop the frit. If the

debris has penetrated the frit and is lodged in the inlet end of the bed itself, then the column has

most likely reached the end of its useful life.

Ion-Exchange Chromatography* [see section: Separations Based on Charge]

This separation mode is based mainly on differences in the ion-exchange affinities of the sample

components. Separation of primarily inorganic ionic species in water or buffered aqueous mobile

phases on small particle, superficially porous, high-efficiency, ion-exchange columns followed by

conductometric or electrochemical detection is referred to as ion chromatography [IC].

Isocratic Elution*

A procedure in which the composition of the mobile phase remains constant during the elution

process.

Liquid Chromatography* [LC]

A separation technique in which the mobile phase is a liquid. Liquid chromatography can be

carried out either in a column or on a plane [TLC or paper chromatography]. Modern liquid

chromatography utilizing smaller particles and higher inlet pressure was termed high-

performance (or high-pressure) liquid chromatography[HPLC] in 1970. In 2004, ultra-

performance liquid chromatographydramatically raised the performance of LC to a new plateau

[see UPLC Technology].

Mobile Phase* [see Eluate, Eluent]

A fluid that percolates, in a definite direction, through the length of the stationary-phase sorbent

bed. The mobile phase may be a liquid [liquid chromatography] or a gas [gas chromatography]

or a supercritical fluid [supercritical-fluid chromatography]. In gas chromatography the

expression carrier gas may be used for the mobile phase. In elution chromatography, the mobile


31/34

phase may also be called the eluent, while the word eluate is defined as the portion of the

mobile phase that has passed through the sorbent bed and contains the compounds of interest

in solution.

Normal-Phase Chromatography*

An elution procedure in which the stationary phase is more polar than the mobile phase. This

term is used in liquid chromatography to emphasize the contrast toreversed-phasechromatography.

Peak* [see Plate Number]

The portion of a differential chromatogram recording the detector response while a single

component is eluted from the column. If separation is incomplete, two or more components may

be eluted as one unresolvedpeak. Peaks eluted under optimal conditions from a well-packed,

efficient column, operated in a system that minimizes bandspreading, approach the shape of a

Gaussian distribution. Quantitation is usually done by measuring thepeak area [enclosed by the

baseline and the peak curve]. Less often, peak height [the distance measured from the peak

apex to the baseline] may be used for quantitation. This procedure requires that both the peak

width and the peak shape remain constant.

Plate Number* [N, see Efficiency]

A number indicative of column performance [mechanical separation power or efficiency, also

calledplate count, number of theoretical plates, or theoretical plate number]. It relates the

magnitude of a peaks retention to its width [variance orbandspread]. In order to calculate a

plate count, it is assumed that a peak can be represented by a Gaussian distribution [a

statistical bell curve]. At the inflection points [60.7% of peak height], the width of a Gaussian

curve is twice the standard deviation [] about its mean [located at the peak apex]. As shown in

Figure U, a Gaussian curves peak width measured at other fractions of peak height can be

expressed in precisely defined multiples of . Peak retention [retention volume, VR, or retention

time, tR] and peak width must be expressed in the same units, because N is a dimensionless

number. Note that the 5 sigma method of calculating N is a more stringent measure of column

homogeneity and performance, as it is more severely affected by peak asymmetry. Computerdata stations can automatically delineate each resolved peak and calculate its corresponding

plate number.

Preparative Chromatography

The process of using liquid chromatography to isolate a compound in a quantity and at a purity

level sufficient for further experiments or uses. For pharmaceutical or biotechnological

purification processes, columns several feet in diameter can be used for multiple kilograms of

material. For isolating just a few micrograms of a valuable natural product, an analytical HPLC

column is sufficient. Both are preparative chromatographic approaches, differing only in scale

[see section on HPLC Scale and Table A].

Resolution* [Rs, see Selectivity]

The separation of two peaks, expressed as the difference in their corresponding retention times,

divided by their average peak width at the baseline. Rs = 1.25 indicates that two peaks of equal

width are just separated at the baseline. When Rs = 0.6, the only visual indication of the

presence of two peaks on a chromatogram is a small notch near the peak apex. Higher efficiency

columns produce narrower peaks and improve resolution for difficult separations;

however, resolution increases by only the square root of N. The most powerful method of

increasing resolution is to increase selectivityby altering the mobile/stationary phase


32/34

combination used for the chromatographic separation [see section on Chemical Separation

Power].

Retention Factor* [k]

A measure of the time the sample component resides in the stationary phase relative to the time

it resides in the mobile phase; it expresses how much longer a sample component is retarded by

the stationary phase than it would take to travel through the column with the velocity of themobile phase. Mathematically, it is the ratio of the adjusted retention time [volume] and the

hold-up time [volume]: k = tR'/tM [see Retention Time and Selectivity].

Note: In the past, this term has also been expressed as partition ratio, capacity ratio,capacity

factor, or mass distribution ratio and symbolized by k'.

Retention Time* [tR]

The time between the start of elution [typically, in HPLC, the moment of injection or sample

introduction] and the emergence of the peak maximum. The adjusted retention time, tR', is

calculated by subtracting from tR the hold-up time [tM, the time from injection to the elution of

the peak maximum of a totally unretained analyte].

Reversed-Phase Chromatography*

An elution procedure used in liquid chromatography in which the mobile phase is significantly

more polar than the stationary phase, e.g. a microporous silica-based material with alkyl chains

chemically bonded to its accessible surface. Note: Avoid the incorrect term reverse phase. [See

Reference 4 for some novel ideas on the mechanism of reversed-phase separations.]

Selectivity [Separation Factor, ]

A term used to describe the magnitude of the difference between the relative thermodynamic

affinities of a pair of analytes for the specified mobile and stationary phases that comprise the

separation system. The proper term is separation factor[]. It equals the ratio of retention

factors, k2/k1 [see Retention Factor]; by definition, is always 1. If = 1, then both peaks

co-elute, and no separation is obtained. It is important in preparative chromatography to

maximize for highest sample loadability and throughput. [see section on Chemical Separation

Power]

Sensitivity* [S]

The signal output per unit concentration or unit mass of a substance in the mobile phase

entering the detector, e.g., the slope of a linear calibration curve [see Detector]. For

concentration-sensitive detectors [e.g., UV/VIS absorbance], sensitivity is the ratio of peak

height to analyte concentration in the peak. For mass-flow-sensitive detectors, it is the ratio of

peak height to unit mass. If sensitivity is to be a unique performance characteristic, it must

depend only on the chemical measurement process, not upon scale factors.

The ability to detect [qualify] or measure [quantify] an analyte is governed by many

instrumental and chemical factors. Well-resolved peaks [maximum selectivity] eluting from high

efficiency columns [narrow peak width with good symmetry for maximum peak height] as well

as good detector sensitivity and specificity are ideal. Both the separation system interference

and electronic component noise should also be minimized to achieve maximum sensitivity.

Solid-Phase Extraction [SPE]

A sample preparation technique that uses LC principles to isolate, enrich, and/or purify analytes


33/34

from a complex matrix applied to a miniature chromatographic bed.Offline SPE is done

[manually or via automation] with larger particles in individual plastic cartridges or in micro-

elution plate wells, using low positive pressure or vacuum to assist flow. Online SPE is done with

smaller particles in miniature HPLC columns using higher pressures and a valve to switch the

SPE column online with the primary HPLC column, or offline to waste, as appropriate.

SPE methods use step gradients [see Gradient] to accomplish bed conditioning, sample loading,washing, and elution steps. Samples are loaded typically under conditions where the kof

important analytes is as high as possible, so that they are fully retained during loading and

washing steps. Elution is then done by switching to a much stronger solvent mixture [see Elution

Strength]. The goal is to remove matrix interferences and to isolate the analyte in a solution,

and at a concentration, suitable for subsequent analysis.

Speed [see Efficiency, Flow Rate, Resolution]

A benefit of operating LC separations at higher linear velocities using smaller-volume, smaller-

particle analytical columns, or larger-volume, larger-particle preparative columns. Order-of-

magnitude advances in LC speed came in 1972 [with the use of 10 m particles and pumps

capable of delivering accurate mobile-phase flow at 6000 psi], in 1976 [with 75-m preparative

columns operated at a flow rate of 500 mL/min], and in 2004 [with the introduction of UPLCtechnology1.7 m-particle columns operated at 15,000 psi].

High-speed analytical LC systems must not only accommodate higher pressures throughout the

fluidics; injector cycle time must be short; gradient mixers must be capable of rapid turnaround

between samples; detector sensors must rapidly respond to tiny changes in eluate composition;

and data systems must collect the dozens of points each second required to plot and to

quantitate narrow peaks accurately.

Together, higher resolution, higher speed, and higher efficiency typically deliver

higher throughput. More samples can be analyzed in a workday. Larger quantities of compound

can be purified per run or per process period.

See #3 on list of References for Further Reading above.

Stationary Phase*

One of the two phases forming a chromatographic system. It may be a solid, a gel, or a liquid. If

a liquid, it may be distributed on a solid. This solid may or may not contribute to the separation

process. The liquid may also be chemically bonded to the solid [bonded phase] or immobilized

onto it [immobilized phase].

The expression chromatographic bedor sorbentmay be used as a general term to denote any of

the different forms in which the stationary phase is used.

The use of the term liquid phase to denote the mobile phase in LC is discouraged. This avoids

confusion with gas chromatography where the stationary phase is called a liquid phase [most

often a liquid coated on a solid support].

Open-column liquid-liquid partition chromatography [LLC] did not translate well to HPLC. It was

supplanted by the use of bonded-phase packings. LLC proved incompatible with modern

detectors because of problems with bleed of the stationary-phase-liquid coating off its solid

support, thereby contaminating the immiscible liquid mobile phase.


34/34

UPLC Technology

The use of a high-efficiency LC system holistically designed to accommodate sub-2 m particles

and very high operating pressure is termed ultra-performance liquid chromatography[UPLC

technology]. The major benefits of this technology are significant improvements in resolution

over HPLC, and/or faster run times while maintaining the resolution seen in an existing HPLC

separation.

For more information, visit:

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